DESCRIPTION (adapted from the Abstract): The overall aim of this competing renewal project is to understand more fully the parasite:host cell interface and, thereby, define key elements that could be used for targets to control toxoplasmosis in AIDS patients. Work in the last funding period has allowed the demonstration that a specific parasite ligand:host cell receptor interaction occurs prior to the process of invasion. The work proposed here is for the identification of this host cell receptor that is unregulated during the S phase of the cell cycle. This gene will be isolated from a cDNA library and compared to other known eukaryotic receptor proteins. The receptor gene will be expressed in insect cells, nonpermissive for T. gondii infection, and the interaction between parasite and receptor studied. In addition, the receptor will be overexpressed or removed from other eukaryotic cells. SAG1 will be expressed in N. caninum to examine the specificity of the host cell receptor for SAG1. The second aim is to explore whether other parasite attachment ligands, in particular the MIC proteins, are important in the process of attachment and activation of human monocytes. Recent observations suggest MIC1 is involved in the interface between parasite attachment and invasion of human monocytes. Studies are proposed to determine whether MIC1 (and perhaps MIC2 and 3) are involved in this process by functional analysis of the MIC protein during the process of invasion and internalization in human monocytic cells. The innate immune response of monocytes stimulated by MIC engagement of the host cell receptor will be evaluated by determining the activation of two signal transduction pathways, induction of cytokines, and the production of a known soluble factor that downregulates host lymphoid proliferation. Strategies directed at interfering with attachment between parasite ligands and host cell receptor(s) should provide novel targets for the development of new therapeutic agents for treating this opportunistic infection in those afflicted with AIDS.